1. Field of the Invention
The present invention relates to a belt driving device and an image forming apparatus provided with the belt driving device.
2. Description of the Related Art
Some conventional image forming apparatuses, such as copiers or printers, form an image by using an endless belt, such as an intermediate transfer belt, a photosensitive belt, or a paper conveying belt. Generally, when an endless belt is run as being stretched over a certain number of rollers including a driving roller, so-called belt meandering, in which a running endless belt is displaced in the direction perpendicular to the direction in which the endless belt is run (hereinafter, “belt-width direction”), may occur. Such belt meandering may cause a distortion in an image, for example, when an image is formed on the outer circumferential surface of the endless belt or on a recording medium carried on the outer circumferential surface of the endless belt. Furthermore, when a color image is to be formed by sequentially forming images of different colors one on top of another, the position of the image of each color may be displaced with respect to the others along the belt-width direction, resulting in color shifts or color unevenness. Because a user can easily detect the color shifts or the color unevenness, it is required to reduce the belt meandering appropriately when a color image is to be formed in the manner describe above.
According to a commonly known technology for reducing the endless belt meandering, an inclination of one or more supporting rollers (hereinafter, “steering roller(s)”) supporting the endless belt is controlled to reduce the belt meandering (hereinafter, “steering method”). In the steering method, a smaller external force is applied to the endless belt compared to a method for reducing the belt meandering by hooking a rib, a guide, or the like arranged on an edge in a belt-width direction of the inner circumferential surface of the endless belt onto an end surface of the supporting roller. Therefore, the steering method is more reliable in running stability and durability of the endless belt.
Japanese Patent Application Laid-open No. H9-48533 and Japanese Patent Application Laid-open No. 2007-3647 disclose conventional belt driving devices using the steering method.
The belt driving device disclosed in Japanese Patent Application Laid-open No. H9-48533 includes an endless driving wire fixed to one end (driving end) of a pre-primary transfer roller (steering roller). The driving wire is moved in a wire-length direction to displace the end (driving end) of the pre-primary transfer roller with respect to the other end to control the inclination of the pre-primary transfer roller. A rotational driving force of a stepper motor is communicated to a driving pulley, among a plurality of pulleys over which the driving wire extend. Because the driving wire is moved by an amount corresponding to the rotation angle of the stepper motor, the amount of the inclination of the pre-primary transfer roller can be controlled by controlling the rotation angle of the stepper motor.
The belt driving device disclosed in Japanese Patent Application Laid-open No. 2007-3647 has a structure including a meandering correcting roller (steering roller) having one end (driving end) attached to one end of a swinging arm, and the other end of the swinging arm is kept in contact with an eccentric cam. The swinging arm is biased by a spring so that the other end of the swinging arm is kept in contact with the cam surface of the eccentric cam. The other end of the swinging arm is displaced as the cam surface is displaced by an amount corresponding to the rotation angle of the eccentric cam. In this manner, the end (driving end) of the meandering correcting roller attached to the end of the swinging arm is displaced relatively to the other end to control the inclination of the meandering correcting roller. Because the rotation angle of the eccentric cam is determined by the rotation angle of a meandering correction motor, the amount of the inclination of the meandering correcting roller can be controlled by controlling the rotation angle of the meandering correction motor.
In the conventional belt driving devices using the steering method, structural members, such as the wire and the eccentric cam, included in an inclining mechanism performing an operation of inclining the steering roller by using a driving force of a driving source may change in the condition, e.g., become worn out, stretched, or contracted, or material thereof may change over a long time of usage. As a result, inclination cannot be controlled properly.
As a specific example, in a wire-based structure in which a wire is used in inclining the steering roller, such as the one employed in the belt driving device described in Japanese Patent Application Laid-open No. H9-48533, the wire has to be tensioned with a tensile force larger than a certain level. Therefore, if the wire is used over a long time, the wire becomes stretched or loose. Furthermore, the wire may also become stretched if an external force of an unexpected strength is applied to the wire. If the wire becomes stretched, the tensile force of the wire is reduced, and the driving force cannot be communicated appropriately from the driving pulley to the wire. Therefore, the operation of the inclining mechanism cannot be controlled appropriately. As a result, the inclination of the steering roller cannot be controlled properly.
If a tensioning mechanism is provided to maintain the tensile force of the wire even when the wire is stretched, the driving force can be communicated appropriately from the driving pulley to the wire even when the wire is stretched. However, in this case, the position of the driving end of the steering roller varies depending on the amount by which the wire is stretched. In other words, even if the operation of the inclining mechanism is controlled to bring the driving end of the steering roller to the same position, the inclination amount of the steering roller differs between before and after the wire is stretched. In this case also, the operation of the inclining mechanism cannot be controlled appropriately, thus the inclination of the steering roller cannot be controlled properly.
In a cam-based structure in which the eccentric cam is used in inclining the steering roller, such as the one employed in the belt driving device described in the Japanese Patent Application Laid-open No. 2007-3647, upon controlling the inclination of the steering roller, the cam surface of the eccentric cam has to slide over a member that moves together with the driving end of the steering roller. Therefore, the cam surface of the eccentric cam or the member kept in contact with the cam surface is worn out and deformed after a long time of usage. Such a deformation also changes the position of the driving end of the steering roller. In other words, even if the operation of the inclining mechanism is controlled to keep the driving end of the steering roller in the same position, the inclination amount of the steering roller becomes different between before and after the deformation. In this case also, the operation of the inclining mechanism cannot be controlled appropriately, and therefore the inclination of the steering roller cannot be controlled properly.
Such a problem may occur in a structure in which the structural member of the inclining mechanism, which performs the operation of inclining the steering roller by using the driving force of the driving source, changes in the condition for some reason, which causes a control error, such as a difference in the actual amount of the inclination of the steering roller between before and after the condition change.